If you’re a huge fan of energy drinks, you might want to listen up: a case study out today shows the perils of drinking too many of these caffeine-rich beverages.

An otherwise healthy 50-year-old man in the UK developed a stroke likely caused by his habit of eight energy drinks per day, according to his doctors. Though the man ultimately recovered from the injury, he continues to have some lingering symptoms years later. The doctors say his case should offer an important lesson that these drinks aren’t necessarily harmless.

“Energy drinks are not currently widely seen as dangerous for cardiovascular risks—they have been discussed because of their effect on sleep and concentration, but the public has little awareness of the more hidden risks they pose,” study author Martha Doyle, a stroke doctor at Nottingham University Hospitals NHS Trust, told Gizmodo.

No other identifiable cause

According to the paper, published Tuesday in BMJ Case Reports, the man visited the hospital with sudden numbness on his left side and general unsteadiness. He also had a remarkably high blood pressure reading of 254/150. For context, 120/80 is normal blood pressure, while 180/120 and above is recognized as a “hypertensive crisis.” Tests soon confirmed he had experienced a mild stroke.

The man was hospitalized for three days. He was discharged with medications to lower his blood pressure and prevent further stroke. Over the next three months of follow-up visits, he appeared to make an excellent functional recovery, though he continued to have some numbness. More worryingly, he still had incredibly high blood pressure despite his medication, and he was even readmitted to the hospital as a result. He was then given more antihypertensives.

Finally, during a more detailed examination of his lifestyle, the man disclosed to doctors that he loved energy drinks. On average, he consumed eight drinks a day, each containing 160 milligrams of caffeine per serving. That amounts to about 1.3 grams of caffeine per day—over three times the maximum recommended dose (400 milligrams) for a typical adult.

The man was fit and healthy for his age, with no major stroke risk factors like a history of smoking or current alcohol drinking. The doctors’ many tests also failed to find any clear precipitating cause for his stroke, other than his very high blood pressure. So given all that, it’s almost certain the man’s energy drink habit played at least a part in spiking his blood pressure and subsequently triggering his stroke.

“We attribute this patient’s stroke to the effects of his energy drink consumption because through thorough investigations using multiple diagnostic methods we ruled out any other identifiable cause,” Coyle said.

Fortunately, the man agreed to quit energy drinks for good. Within a week, his blood pressure started to return to normal, and it remained healthy after he was weaned off the antihypertensives (yet more evidence that the drinks were to blame). Further follow-up visits showed that he made a complete recovery with no further strokes, and eight years later, his blood pressure continues to be normal.

That said, he does still experience some residual numbness to this day.

“I obviously wasn’t aware of the dangers drinking energy drinks were causing to myself, (I) have been left with numbness (in my) left hand side hand and fingers, foot and toes even after eight years,” the man wrote in a patient perspective accompanying the report.

A needed warning

The doctors say that there are several reasons why energy drinks can raise the risk of stroke and other cardiovascular problems, mostly tied to the high amounts of caffeine and sometimes sugar found in them.

Aside from causing a chronic rise in blood pressure, for instance, too much caffeine might cause a sudden narrowing of blood vessels in the brain or trigger an arrhythmia (an irregular heartbeat). Some ingredients popular in these drinks, such as taurine and ginseng, might also amplify the effects of caffeine, while others like guarana can contain high amounts of undisclosed caffeine.

There have been some high-profile cases of cardiovascular disease tied to energy drinks. Between 2022 and 2023, for instance, at least two heart-related deaths in the U.S. were linked to Panera Bread’s highly caffeinated “Charged Lemonade” energy drinks. By mid-2024, Panera had removed the item from its menu, and earlier this summer, the company settled the remaining lawsuits over these deaths and other reported injuries.

But the report authors say that—much like their patient—the public still isn’t generally aware of the health risks these drinks can carry. And it’s possible that these risks could warrant increased regulation of these products or the advertising for them. Compared to something like alcohol, they note, energy drinks are frequently marketed to younger kids.

“Whether health authorities should be actively intervening we cannot definitively conclude from our reporting of just this case,” Coyle said. “However it would be beneficial to get this out there so if any similar cases exist they can be brought to attention. And also for the public, they have the right to know the risks of what they are consuming.”

Speaking as someone who loves his morning and sometimes afternoon coffee, I’ll certainly be a bit more proactive in tracking my daily caffeine consumption from now on.